Joint parts employed in a vehicle suspension include a yoke 43 shown in FIG. 2, which has a plurality of branches 21, 22 and 23.
Conventionally, the yoke has been formed, as shown in FIG. 3, by forging a solid round bar 31 that is a material to be subjected to forging into a forged product having a flash 32 formed at its periphery.
Alternatively, as shown in FIG. 4, a product 43 (i.e., a yoke) has been formed by subjecting to mechanical machining a portion 42 of a yoke preform 41 that has been extruded and cut so as to assume a shape substantially similar to that of the product.
Recently, instead of iron, aluminum alloy has been increasingly used for producing suspension parts for vehicles in order to reduce the weight of the parts. The suspension parts for vehicles have been produced through forging in order to enhance their mechanical strength and to reduce the amount of raw material used for producing a product. Examples of the suspension parts for vehicles include an upper arm and a lower arm.
Since an upper arm 54 shown in FIG. 5, which is a suspension part for a vehicle, has branches 51, 52 and 53 extending in three directions, production of the upper arm in a single forging step is difficult. Therefore, conventionally, the upper arm has been produced by producing an upper arm preform 61 as shown in FIG. 6 having a shape similar to that of a final product through forging and subjecting the preform to a plurality of forging steps to thereby cause the preform to assume the shape of the upper arm 54 shown in FIG. 5.
Specifically, a solid round bar 71 as shown in FIG. 7 is subjected to forging by use of a forging die, and then a flash 72 is removed from the forged body by use of a trimming die to thereby produce an upper arm forging preform 73. Subsequently, the preform 73 is subjected to a plurality of forging steps to thereby obtain a vehicle upper arm 74. In this case, in order to reduce loss of the material incurred by the formation of the flash, there is employed a forging die having a configuration allowing a plurality of upper arm preforms 73a to be produced from one solid round bar material in a single step.
Meanwhile, a closed forging method in which-no flash is formed is known as a method for forging a disk-shaped material 82 into a product of simple form, which is, for example, a simple circular or cylindrical product such as a VTR cylinder 81 shown in FIG. 8.
JP-A HEI 1-166842 discloses a method for producing through closed forging a product having a plurality of branches. In the method disclosed in this publication for producing a product having a plurality of radially extending branches, as shown in FIG. 9, a punch 91 is used to apply pressure to a solid round bar material so as to fill impressions provided in upper and lower dies 93 and 94 to thereby form radially extending branches 92 through closed forging.
The conventional method shown in FIG. 3, in which flashes are formed, requires a trimming step for removing the flashes subsequent to a forging step. In such a method, since unwanted flashes are formed around a forged body, the yield of the product on the basis of a forging material is low. In addition, since the projection area of the forged body as viewed in the direction perpendicular to the direction of pressure application is large, a large, expensive forging machine capable of applying high load is required, resulting in high production cost.
Also, in the conventional method shown in FIG. 4 in which the preform 41 which has been obtained through cutting of an extruded material is subjected to machining to thereby produce a yoke 43, since the portion 42 is subjected to machining, a large allowance must be provided for machining, and as a result, the yield of a final product on the basis of the preform is low. In addition, the method requires steps for machining, resulting in high production cost.
The aforementioned conventional method for producing a preform of an upper arm or lower arm, which is a suspension part for a vehicle, requires a trimming step for removing flashes subsequent to a forging step. In this method, since unwanted flashes are formed around the preform, the yield of the preform on the basis of the material is low. In addition, since the projection area of the forged body as viewed in the direction perpendicular to the direction of pressure application is large, a large, expensive forging machine capable of applying high load is required, resulting in high production cost.
In the closed forging method disclosed in JP-A HEI 1-166842, pressure is applied in a direction perpendicular to the cut surface of a cylindrical material so as to cause plastic flow of the material, thereby forming radially extending branches 92. Therefore, when the branches 92 are long or fail to attain uniform length (i.e., the branches have different shapes), forging defects such as underfill and overlap on the surface of a forged product may be generated, because of the difference in the rate or direction of plastic flow of the material between portions of the forged product.
In view of the foregoing, one object of the present invention is to provide a closed forging method for producing a member having a plurality of branches, in which load applied to a raw material is reduced during forging and the yield of a final product on the basis of the raw material is improved; a forging production system employing the method; and a forging die employed in the method and system.
Another object of the present invention is to provide a method for efficiently producing a suspension part for vehicles and a preform or yoke thereof at low cost.
Still another object of the present invention is to provide a forged product of high mechanical strength, which is produced by causing plastic flow of a forging material to occur along a plurality of branches to thereby form layers of metal flow in the branches.
The term “materials” used throughout the description refers to articles not forged, which include ingots, forging materials, cut pieces, solid round bars, raw materials, cylindrical materials, continuous forging round bars, disks and billets.
The term “preforms” used throughout the description refer to products obtained by forging but required to undergo at least one further forging step into a final product, which include yoke performs, upper arm performs and upper arm forging performs.
The term “forged products” used throughout the description refers to products having been forged, which include members, products, final products, forged bodies and forged articles.